1. Field of the Invention
This invention relates to a method and apparatus for restoring operation to cells within a radiocommunication network after a system disturbance, and more particularly, to a method and apparatus for scheduling recovery of cells within a radiocommunication network based on the anticipated traffic density within each cell immediately following recovery.
2. Discussion of Related Art
An exemplary cellular radio communication system is illustrated in FIG. 1. As shown there, a geographic region served by the system is subdivided into a number of cells, each containing a base station. In this exemplary case, there are ten cells C1.gtoreq.C10 including base stations B1-B10, respectively. Although the cells are shown as having a hexagonal shape, this is merely a graphical convention; in actuality, the cells may have a variety of "shapes" depending on the terrain covered by the cell and the characteristics of the antenna used by the cell's base station. Although not shown, each cell is also typically divided into a plurality of sectors which are serviced by a respective plurality of base station sector antennas.
Mobile terminals M1-M10 are shown scattered throughout the geographic coverage area encompassed by cells C1-Cl0. Each mobile terminal typically communicates with its nearest base station, using, for instance, frequencies assigned to that base station. The base stations of adjacent cells use different frequencies so as to reduce interference between neighboring cells. When a mobile terminal moves from one cell to another, the system will transfer any ongoing call to the new cell. Alternatively, in the Code Division Multiple Access (CDMA) protocol (e.g., in the IS-95 standard), all cells share a common wide-band channel. In this protocol, a mobile terminal can simultaneously communicate with plural base stations during handoff from one cell to another.
As illustrated in FIG. 1, base stations B1-B10 are coupled to a mobile services switching center (MSC), which provides a connection to a public switched telephone network (PSTN) (not shown), which, in turn, provides connection to various communication devices, such as a telephone handset, terminal, etc. (not shown).
Various safeguards can be taken to ensure the continuity of radiocommunication service provided by the network. Nevertheless, interruptions in service can and do occur. These interruptions may be due to the deliberate actions of a system operator, or may be caused by various system failures. For instance, planned interruptions may be used to perform maintenance on the system or to make upgrades. Failures may be attributed to transmission and other processing errors, power failures, component failures, etc. Whatever the cause, these interruptions result in a temporary blocking of communication services provided by the entire system or parts thereof. In the following discussion, the term "disruption" (or "disturbance") is used to describe these interruptions in service, which encompasses any type of planned or unplanned interruption in service.
After a disruption, the communication service provided in each cell must be restored. Cells are typically restored sequentially, such that operation is restored to the network on a cell-by-cell basis. (This is because, at the present time, restoring all of the cells at the same time is not feasible, because it would impose a large processing burden on the system.) In one technique, operation is restored to the cells in a random order, such that no cell is given priority over other cells when powering up the cells. In a second technique, cells are restored according to a fixed schedule.
The above-described techniques for restoring operation to the radiocommunication cells have a number of drawbacks. With reference to FIG. 1, traffic may differ from cell to cell. For instance, there may be more traffic in cell C1 than in adjacent cell C2 because, for instance, cell C1 corresponds to a heavily populated urban area while cell C2 corresponds to a more rural area. In the first mentioned technique, power is restored on a random basis, such that any one of cells C1-C10 is powered up first. This means that the busiest cell, C1, may not be powered up first. This can result in a loss of revenue for the system operator, since the cell with the greatest potential demand for service (e.g., the potential number of calls) is not necessarily restored first. Also, users of mobile terminals are understandably disturbed when they are unable to initiate or receive calls during a system disturbance. Restoring operation to cells in a random order does not minimize this disturbance, since, again, the cell with the greatest number of potential calls is not necessarily restored first.
The second technique addresses some of these concerns by, for instance, allowing the system to restore power to a cell which serves an urban area before a cell which serves a rural area. Yet this provision may not be enough to account for the complexity in traffic patterns within a coverage area. As mentioned, the exemplary cell C1 corresponds to an urban area, and therefore may have greater traffic than adjacent rural cell C2. However, assume that cell C1 correspond to a business district within a city which has heavy call traffic during normal working hours, but otherwise has relatively low traffic. Accordingly, cell C1 may have heavier call traffic than cell C2 during the middle of the day, but the traffic of cell C2 might exceed that of cell C1 at other times. For instance, if cell C2 included a major roadway into the urban area C1, the traffic in cell C2 could conceivably be higher than the traffic in cell C1 during "rush" hours. Similar variable density patterns may be present in other cells, which may be attributed to a variety of reasons, including special unpredictable events which result in an entirely anomalous traffic pattern throughout the coverage area. The end result is that it is difficult to predict which cell may be busiest at any moment, making the second mentioned recovery technique also unsatisfactory.
Hence, it is a general objective of the present invention to provide a method and system for re-establishing operation within cells in a more "intelligent" manner so as to provide more efficient management of the radiocommunication system.